Electronic structures and dynamics of the excited triplet states of α,ω-diphenylpolyynes

The lowest excited triplet $(T1)$ states of diphenylacetylene and several α,ω-diphenylpolyynes (DPY) having two, three, four and six triple bonds were studied using continuous wave time-resolved electron paramagnetic resonance (CW-TREPR), pulsed EPR, and phosphorescence spectroscopy. Linear and planar molecular structures in the $T1$ states of DPY were suggested from the magnetophotoselection experiments and observation of a strong 0-0 band in the well-resolved phosphorescence spectra. The spin density distributions, which were obtained by electron spin echo envelope modulation measurements and semi-empirical molecular orbital calculations, and the phosphorescence polarization normal to the long axis of molecule for the 0-0 bands suggested that the $T1$ states of DPY have a $B1u3(πxπx*)$ electronic configuration. DPY showed an abnormally large $|D|$ value of the zero-field splitting parameters that increased with increasing molecular size. Such an unusual behavior of the D values is interpreted in terms of the spin-orbit interaction between the $T1$ and $Au3(πxπy*)$ states mainly due to their energy proximity which is probably characteristic of the excited states for the polyynes. The estimated energy gap between the $πx3πy*$ and $πx3πx*$ states for DPY ranges from 2900 cm⁻¹ to 1400 cm⁻¹. There existed a good correlation between the acceleration of the nonradiative decay rate constant from the $T1$ state and appearance of a ring twisting vibronic band with $b1g$ symmetry in the phosphorescence spectra for DPY. Therefore, we concluded that the vibronic interaction between the $πx3πy*$ and $πx3πx*$ states promoted by the $b1g$ vibration leads to a broadening of the potential surface of the $T1$ state which results in the fast radiationless decay to the ground state.